Applying the method originally developed in the theory of quantum semiconductor
superlattices [1]}, an exact solution of equation (6) can be constructed
without assuming a weak electric field. By expanding the distribution functions
of interest in Fourier series, we have:
\(f(p,t) = \Delta p_{\vartheta } \sum _{s=1}^n{\delta (p_{\vartheta } - s\Delta p_{\vartheta })}\sum _{r \ne 0}{f_{rs}exp({iarp})\psi _v(t)}\) 
